The possibility exists that some therapeutic proteins, such as antibodies, have immunogenic potential, and administration of the therapeutic proteins to a patient sometimes results in the production of antibodies against the therapeutic protein. Such anti-drug antibodies (ADA) may reduce the effectiveness of the therapeutic protein; for example, they may bind to or/and neutralize the therapeutic protein, resulting in changes of drug pharmacokinetics or pharmacodynamics that alters drug efficacy. ADA may cause serious side effects, including allergic reactions, cross-reactivity against endogenous proteins, and complement activation. A life-threatening deficiency syndrome can result if ADA cross-reacts with and neutralizes a critical endogenous protein. Thus, assays and reagents for determining the presence of ADAs in a sample for a patient who has received an antibody therapy, such as pembrolizumab, is of great interest.